These studies seek to assess the effects of environmental agents on oogenesis and spermatogenesis. Environmental chemicals are tested, test systems validated and new approaches to toxicity testing proposed. To date, alkylating agents and selected trace metals have been studied. The DNA repair system in mouse germ cells appears useful as a predictor of toxicity. Biochemical and toxic effects of environmental agents on rodent spermatogenesis are also studied using velocity sedimentation cell separation and serial mating techniques. Enzyme development during spermatogenesis and "marker" enzymes have been defined which indicate normal cellular differentiation as well as toxic effects of chemicals. The nature of the blood-testis barrier and the penetration of environmental chemicals from blood to spermatogenesis cells have been reported. These studies contribute to an improved understanding of the reliability of laboratory animal toxicity tests for reproductive effects and the validity of their extrapolation to humans.